The present invention relates to an apparatus for preparing bituminous mixtures, especially road construction mixtures which include broken-up road material.
The present type of apparatus is used for mixing, heating, and drying aggregate and bituminous binder components preferably in a continuous manner. However, this type of apparatus is also suitable for the batch type preparation of bituminous mixtures. The mineral aggregates still comprise a certain moisture proportion when they are supplied into the apparatus whereby dust production is minimized. Such devices comprise a kiln type rotary drum through which the components of the mixture and the heat travel in the same direction from a feed-in end to a discharge end. The rotary drum is divided into several zones constructed for different purposes. German Pat. No. 2,102,328 first published on Aug. 3, 1972 and granted on Sept. 30, 1975 discloses an apparatus of the just described type.
These mixing devices have been found to be very practical. However, in those instances where the aggregate comprises components obtained from breaking up the surface of roads that require repairs, the just described apparatus leaves room for improvement. It has been found that the prior art structure does not distribute the broken-up road components such as broken-up road surface layers and broken-up road binder layers in a sufficiently uniform manner. However, the success of reusing broken-up road material as a component in the new mixture depends on the uniform distribution of the broken-up material throughout the volume of the new mixture.
German Pat. No. 1,594,815 describes an apparatus for mixing heating and drying bituminous mixtures in a continuous or batch type operation. In this apparatus the mineral aggregate components also still comprise some moisture at the time of joining these mineral components with the bituminous binder material. In this known apparatus the non-dried mineral components and the binder components are mixed until a sufficient statistic distribution of the binder components on the surfaces of the mineral components has been achieved. Thereafter the so premixed material is supplied into a dryer where it is sufficiently heated. This type of heating requires a careful classification of the individual fractions of the aggregate components, each of which requires its particular moisture content for the successful mixing of the total mineral components with the bituminous binder components.
The apparatus of German Pat. No. 1,594,815 is suitable for the batch type mixing only. Further, the broken-up road material added to the aggregate includes already bituminous binder material which is not sufficiently plastified at the time of mixing with the new aggregate components. Therefore, there is the possibility that once the binder material in the broken-up road material becomes plastified there may be formed zones which have too much binder material. The accumulation of undesirably high proportions of binder material in certain zones of the mixture is due to the fact that the binder material of the broken-up aggregate component combines with the freshly added binder material in such zones. The accumulation of excessive binder material is undesirable because it hinders the transport of the premixed material to the dryer by sticking to the containers used for the intermediate transport. Thus, a demixing may prevent the desired uniform distribution of the mixture components throughout the volume of the mixture.